It has been known in the prior art that butadiene polymers having a high content of trans-linkage can be produced by the techniques which may be classified into the three categories as described below.
That is, they can be produced according to the production techniques employing (1) the so-called Ziegler catalyst system comprising a transition metal compound as the main component, (2) the anionic polymerization catalyst system comprising an alkaline earth metal compound as the main component, and (3) the catalyst system comprising a rare earth metal compound as the main component.
The first technique employing a transition metal such as nickel, cobalt, titanium, vanadium, etc. as the main component is known to effect highly stereoregular polymerization. For example, as the method for polymerization of butadiene by using titanium metal, there is the method in which a tetravalent titanium metal compound and a carrier of a magnesium halide are employed (Japanese Laid-open Patent Publication No. 67387/1976). In the case when employing a vanadium compound as the main component, a polymer having a very high content of trans-linkage can be obtained. For example, there have been known the method in which isoprene is polymerized by using a complexed catalyst comprising tetravalent vanadium halide and an organic aluminum (Japanese Laid-open Patent Publication No. 36585/1975), and further the method in which isoprene is polymerized by using a complexed catalyst comprising a trivalent or tetravalent vanadium compound, an organic aluminum and a tetravalent titanium compound (Japanese Laid-open Patent Publications Nos. 29386/1974 and 122586/1975). However, these catalysts have the drawbacks that they are generally insoluble in hydrocarbon solvents and also provide polymers which are likely to become highly branched with the progress of polymerization, and the polymers obtained have a markedly broad molecular weight distribution. Such polymers are inferior in processability and moldability, and also disadvantageously inferior in physical properties such as resilience, strength and impact resistance of the molded articles obtained.
On the other hand, as the method belonging to the above category (2), there is an example in which an organic metal compound of IIA metal is used as the polymerization catalyst. However, generally speaking, organic metal compounds of IIA metals other than beryllium and magnesium can be synthesized with difficulty, and their activities for polymerization of conjugated dienes are markedly low. In the case of organic metal compounds of beryllium and magnesium, while they can be synthesized with relative ease, they have no activity for polymerization of conjugated dienes except under some special reaction conditions, and there is no example of practical application. Whereas, the method in which a complexed catalyst comprising a combination of an organic compound of IIA metal with another organic metal compound is used, may be exemplified by polymerization of butadiene by using barium-di-tert-butoxide and an organic lithium (U.S. Pat. No. 3,992,561) or barium-di-tert-butoxide and an organic magnesium (U.S. Pat. No. 3,846,385). Further, it has also been known to carry out the polymerization of conjugated dienes by using an organic compound of barium or strontium, an organic lithium and an organic metal compound of IIB or IIIA metal (U.S. Pat. No. 4,092,268). In the systems employing the complexed catalysts comprising these IIA compounds, conjugated diene polymers having a somewhat high content of trans-linkage can be obtained, having the molecular weight distribution being also relatively narrower. However, when it is desired to obtain a polymer having a still higher content, for example as high as 80%, of trans-linkage in the diene moiety, it is generally required to employ a lower polymerization temperature. In this case, the polymerization activity of these catalyst systems will become markedly lower and unsatisfactory for industrial application.
Under such special conditions, polymers with high content of trans-linkage, and narrow molecular weight distribution can also be obtained. However, these polymers are markedly lower in 1,2-linkage content as compared with the polymers of the present invention and, probably because of the difference in crystalline structure between the polymers due to such a difference, they were inferior in physical properties, particularly in impact resistance, of the polymers.
Further, as the catalyst belonging to the above category (3), a complexed catalyst employing a rare earth metal compound as the main catalyst and an organic magnesium compound as the co-catalyst is also known. For example, in European Patent No. 0,091,287, there is proposed the method in which a Versatic Acid salt such as of Di, Nd, Pr, etc. is used. However, such a complexed catalyst involves the drawbacks such that it is very low in its polymerization activity, also the polymer obtained has a low molecular weight, and further that the molecular weight distribution is broad, thus providing no polymer having physical properties for practical use.
As described above, although various production techniques have been known in the art, there has been known no method for obtaining a crystalline trans-butadiene polymer having a high content of trans-linkage, a narrow molecular weight distribution and containing substantially no gel at levels utilizable for industrial application. Also, the trans-butadiene polymers obtained in such prior art techniques involved problems in that they were inferior in moldability and workability, and also inferior in balance between physical properties such as rigidity, strength, resilience, impact resistance, etc.
Solution polymerization of butadiene in a hydrocarbon solvent has been carried out by using a complexed catalyst comprising (a) an organic acid salt of lanthanum or cerium and (b) an organic magnesium compound, and consequently found that a crystalline trans-butadiene polymer having a high trans-linkage content, a narrow molecular weight distribution and containing substantially no gel, can be obtained, and further that the polymer obtained can be easily molded and worked at a relatively lower temperature, is excellent in miscibility with inorganic fillers or other various resins, and also excellent in physical properties such as regidity, strength, resilience, impact resistance, etc., in either a vulcanized or unvulcanized state, and thus have accomplished the present invention.